Conventionally, terrestrial digital television broadcasting and satellite broadcasting have been implemented using dedicated forward link channels. In a recently developed service, television programs or other contents are distributed to multiple terminals using multicast communications.
Meanwhile, mobile terminals such as portable phones have been used as the receiving terminals of such broadcasts or distributions. A problem associated with such mobile terminals is that reception quality of the broadcast or distributed data varies from one place to another, resulting in poor picture quality.
In view of such a drawback, Japanese Laid-Open Patent Publication No. 2004-186892 (published on Jul. 2, 2004), and No. 2004-289830 (published on Oct. 14, 2004) disclose techniques in which a transmitting station transmits a packet more than one time. This is intended to improve the probability of packets being properly received by the receiving terminals and thereby improve reception quality.
Japanese Laid-Open Patent Publication No. 2001-308784 (published on Nov. 2, 2001) discloses the following technique. As disclosed in this publication, a transmitting station or a receiving terminal determines the reception quality. If there is deterioration in the quality of the radio frequency communications, the receiving terminal is disconnected from the multicast group and the communication between the receiving terminal and the transmitting station is switched to point-to-point communications which involve retransmission. If the quality of the radio frequency communications improves later on, the receiving terminal is reconnected to the multicast group, and the communication between the receiving terminal and the transmitting station is switched to multicast communications, which are point-to-multipoint communications involving no retransmission. In this way, when the quality of radio frequency communications is poor, the receiving terminal sends a request for retransmission of unreceived data to the transmitting station using point-to-point communications involving retransmission (bidirectional communications). Since the data is retransmitted, the quality of received data improves at the receiving terminal.
Japanese Laid-Open Patent Publication No. 2002-10157 (published on Jan. 11, 2002), and No. 2004-128563 (published on Apr. 22, 2004) disclose a receiver which sends a request for retransmission of a broadcast signal to the transmitting station using a bidirectional channel when the quality of the broadcast signal transmitted on a dedicated forward link channel is poor, so as to receive retransmitted packets.
However, a problem of transmitting the same packets multiple times is that it does not allow for efficient use of the communication channels. That is, it involves a large amount of unnecessary communications.
Further, in response to the request for retransmission using a bidirectional channel, the transmitting station needs to process the request for each receiving terminal, with the result that a large burden is put on the transmitting station. Further, due to the time lag associated with the requesting and processing of the retransmission, the receiving terminal receives the retransmitted packet with a delay, as compared with the case where the packet is properly received without the request for retransmission. This may cause interruption or disturbance of video data when the data contains a real time video image that is reproduced as it is received. Another problem is that it may congest the bidirectional channel, with the result that the communication speed of the retransmitted data may be decreased, or interference may be caused in sending and receiving other data.
Further, since the receiving terminals need to send a retransmission request individually, the burden on the receiving terminals is increased.